Late Jurassic Climates, Vegetation, and Dinosaur Distributions
P. McAllister Rees, Christopher R. Noto,1 J. Michael Parrish,2 and Judith T. Parrish3
Department of Geosciences, University of Arizona, Tucson, Arizona 85721, U.S.A.
(e-mail: rees@geo.arizona.edu)
ABSTRACT
The Jurassic and Cretaceous are considered to have been warmer than today on the basis of various climate data and
model studies. Here, we use the available global record of climate-sensitive sediments, plants, and dinosaurs to infer
broadscale geographic patterns for the Late Jurassic. These provide a context for our more detailed accounts of the
Morrison and Tendaguru Formations in North America and East Africa. At the global scale, evaporites predominated
in low latitudes and coals in mid- to high latitudes. We ascribe these variations to a transition from drier to wetter
conditions between the equator and poles. Plant diversity was lowest in equatorial regions, increasing to a maximum
in midlatitudes and then decreasing toward the poles. Most dinosaur remains are known from low-latitude to marginally midlatitude regions where plant fossils are generally sparse and evaporites common. Conversely, few dinosaur
remains are known from mid- to high latitudes, which have higher floral diversities and abundant coals. Hence, there
is an obvious geographic mismatch between known dinosaur distributions and their primary food source. This may
be due to taphonomic bias, indicating that most dinosaur discoveries provide only a small window on the diversity
and lifestyles of this group. On the basis of our global- and local-scale studies, we suggest that dinosaur preservation
was favored in environments toward the drier end of the climate spectrum, where savannas rather than forests
predominated. A holistic approach, incorporating climate and vegetation as well as geography, is required to better
understand patterns of dinosaur ecology and evolution.
Introduction
seasonally summer wet or winter wet, midlatitudes
were mostly warm temperate, and high latitudes
were cool temperate. Climate was more equable
than today, in the sense that there were no polar
icecaps and the equator to pole temperature gradient was lower, but vegetation was still far from
uniform. There is no evidence of tropical rainforest
in the Jurassic; indeed, limited precipitation was
the main restriction on plant growth in low latitudes. Although the poles were warmer than today,
continuous plant growth there would nevertheless
have been constrained by the onset of winter
darkness.
The climate data and model results match well
in low and midlatitudes although unresolved discrepancies remain in the high latitudes, where the
models produce temperatures colder than those estimated by the plant data (e.g., model cold temperate vs. data cool temperate; Rees et al. 2000).
This problem has been encountered in all other
“hothouse” climate models, which consistently
predict cold conditions in high-latitude regions beyond the tolerance limits indicated by the plants
Not surprisingly, dinosaurs arouse intense scientific and public curiosity. Spectacular skeletons, supreme dominance of the terrestrial realm for ∼150
m.yr., and scary movies all ensure their role as the
subject of serious study as well as popular titillation. However, relatively little is known of dinosaurs in the broader context of climate, geography,
and their primary food source, vegetation. Here, we
describe distribution patterns of Late Jurassic dinosaurs in that context. Late Jurassic climates have
been interpreted from data and model studies (e.g.,
Vakhrameev 1991; Hallam 1994; Rees et al. 2000;
Sellwood et al. 2000) and vegetation patterns outlined (Vakhrameev 1991; Rees et al. 2000). In essence, low-latitude regions were either desert or
Manuscript received February 24, 2004; accepted June 9,
2004.
1
Department of Ecology and Evolution, State University of
New York, Stony Brook, New York 11794, U.S.A.
2
Department of Biological Sciences, Northern Illinois University, DeKalb, Illinois 60115, U.S.A.
3
College of Science, University of Idaho, Moscow, Idaho
83844, U.S.A.
[The Journal of Geology, 2004, volume 112, p. 643–653] 䉷 2004 by The University of Chicago. All rights reserved. 0022-1376/2004/11206-0003$15.00
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P. M . R E E S E T A L .
(e.g., the Permian; Rees et al. 2002), even for time
intervals such as the Late Cretaceous, where quantitative climate estimates have been derived from
angiosperm leaf physiognomy (e.g., Spicer et al.
1996; Herman and Spicer 1997).
For the purposes of this article, we focus on the
low- and midlatitude regions, simply because it is
there that Late Jurassic dinosaur remains are best
known. The preceding discussion does, however,
raise the question of whether the poleward latitudinal limits of these dinosaur remains is due to
climate tolerances of the living animals or taphonomic biases. We address this later, but first we
concentrate on the Late Jurassic record of plants,
climate-sensitive sediments, and dinosaurs. The
global-scale aspects of this study expand on earlier
work with A. M. Ziegler on “Correspondence of
paleolatitudinal diversity in herbivorous dinosaurs
and plants during the Jurassic” (Parrish et al. 1996).
Late Jurassic Global Distribution Patterns
The known paleogeographic distributions of Late
Jurassic plants, climate-sensitive sediments, and dinosaurs are described and illustrated here. These
are then considered in the context of latitudinal
gradients in taxonomic diversity, a well-studied
phenomenon in modern biota (Rosenzweig 1992,
1995; Gaston 2000). Present diversity is highest
around the equator within tropical regions and decreases toward the poles. There have also been
many published accounts of latitudinal gradients
in the fossil record (Crane and Lidgard 1989; Kiessling 2002), lending credence to the idea that latitudinal gradients in diversity have been consistent
features throughout Earth’s history.
Plants and Climate-Sensitive Sediments. The floral
and climate-sensitive sediment data used in this
study were compiled by A. M. Ziegler and the Paleogeographic Atlas Project. Applying these data,
Rees et al. (2000) interpreted Jurassic vegetation
patterns. Plant diversity was lowest at the equator
and poles but reached a maximum in each hemisphere at the midlatitudes. This is in marked contrast to today, where maximum plant diversity occurs in the equatorial regions. However, measures
of diversity provide only a partial insight; it is also
important to know the dominant vegetation of each
region. For example, boreal conifer forests today
have relatively low diversity but high productivity.
In the Jurassic, maximum plant diversity (and, apparently, productivity) was concentrated at midlatitudes, where forests were dominated by a mixture of conifers, cycadophytes, pteridosperms,
ferns, and sphenophytes. Low-latitude vegetation
tended to be xeromorphic (i.e., dry-adapted) and
only patchily forested, typified by the presence of
small-leafed (microphyllous) forms of conifers and
cycadophytes. Polar vegetation was dominated by
large-leafed (macrophyllous) conifers and ginkgophytes that were apparently deciduous. Tropical everwet vegetation was, if present at all, highly restricted. Hence, studies of diversity address only
part of the question; it is also important to understand the nature of the contributing organisms.
Late Jurassic plant localities, scaled according to
the number of genera in each, are shown on a paleogeographic map in figure 1A. Analysis was restricted to relatively bona fide Late Jurassic floras,
so, for example, localities in Argentina, East Antarctica, and eastern Australia were excluded (cf.
Rees et al. 2000). There is a paucity of localities
between 30⬚N and 30⬚S and a high number of genera
from localities in present-day India, China, eastern
Russia, and Europe. These patterns are instructive
in that they show not only the incomplete nature
of the geographic record but also some latitudinal
control on plant distributions and diversity. However, the plant types preserved in these localities
are more significant than the raw numbers of genera in each. Microphyllous forms of conifers and
cycadophytes, as well as pteridosperms, tend to be
most abundant at low to midlatitudes, along with
sphenophytes and ferns. Progressing poleward,
ferns, macrophyllous cycadophytes, and sphenophytes become more abundant in mid- to high latitudes, with ginkgophytes and macrophyllous conifers most abundant in the high latitudes.
Significantly, low-latitude plant localities have not
only relatively low plant diversity but also relatively high proportions of microphyllous taxa. Midto high-latitude ones have relatively high diversity
as well as more macrophyllous taxa.
Climate-sensitive sediments such as coals and
evaporites provide useful indicators of the precipitation/evaporation ratio, indicating relatively wet
or dry climate regimes, respectively. Their Late Jurassic distributions are shown in figure 1B, from
which it is clear that most evaporites occur at low
latitudes between ca. 40⬚N and 40⬚S of the equator,
whereas most coals occur poleward of these limits.
This coincides with the floral patterns; low-diversity microphyllous-dominated floras occur in low
latitudes, and higher-diversity macrophyllousdominated ones occur in the mid- to high latitudes.
The coal and evaporite patterns shown here are not
unique to the Late Jurassic (although, of course,
peats occur today in low as well as mid- to high
latitudes). Indeed, on the basis of these and other
lithologic indicators of climate, Ziegler et al. (2003)
Figure 1. Late Jurassic (150 Ma) paleogeographic maps (Mollweide projection with 30⬚ latitude and longitude lines).
A, Plant localities, scaled according to the number of constituent genera. B, Evaporite and coal deposits. C, Dinosaur
localities, scaled according to the number of constituent taxa and showing the location of the Morrison (M) and
Tendaguru (T) Formations.
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P. M . R E E S E T A L .
Figure 2.
Distributions of dinosaur taxa (A) and plant genera (B) in 10⬚ paleolatitudinal bins
have documented the latitudinal constancy of climate zones from the Permian to present, relating
it to Hadley cell circulation.
Dinosaurs.
The topic of dinosaur distribution
and diversity has been discussed by paleontologists
almost since their initial discovery (see Dodson
1997; Forster 1999 for reviews). Most studies have
focused on biogeography, relating continental configurations and tectonic histories to patterns of dinosaur distribution and evolution (e.g., Kalandadze
and Rautian 1991; Russell 1993; Dodson 1997; Forster 1999; Sereno 1999a, 1999b). Attention has also
been focused on dinosaurs in the context of ecology
as well as biogeography, and the ways in which
environmental differences may have determined
their distributions (e.g., Lehman 1987, 2000; Parrish et al. 1996; Foster et al. 2001; Noto et al. 2002).
Using the methods for reconstructing paleoclimate and plant distributions mentioned previously,
it is now possible to associate dinosaurs with these
general climatic regimes, or biomes (see Rees et al.
2000 for details). By assessing dinosaur distributions and diversity in the context of climate and
geography, we can gain insights into their paleoecology and the evolutionary pressures exerted by
changes in ecological and climatic conditions. Late
Jurassic dinosaur data used in this study are from
the upcoming second edition of The Dinosauria
(Weishampel et al. 2004). Remains consist of bones,
teeth, tracks, eggs, or coprolites. The last two were
excluded from analysis because of the difficulty in
assigning these remains to any particular taxon, as
well as the possibility that these remains may not
belong to dinosaurs at all. Taxonomic diversity was
calculated by counting body fossils (bones and
teeth) and/or trace fossils (tracks) of taxa within
each dinosaur-bearing formation. Fossils unidentifiable to the genus level (e.g., some indeterminate
higher taxa or tracks) were counted in a formation
only when a representative of that group was not
already present. For example, if the genus Allosaurus was present, then neither “indeterminate theropod” nor “theropod tracks” was counted for that
formation.
Figure 1C shows the geographic distribution of
Late Jurassic dinosaur remains. From a perusal of
figure 1, it is evident that dinosaur fossils are restricted to between ca. 45⬚N and 45⬚S of the paleoequator and that this distribution broadly coincides
with that of evaporites. High-diversity floral localities do occur at the poleward extremes of dinosaur
distributions but, as mentioned earlier, most of the
floras where dinosaurs occur have relatively low
diversity and comprise higher proportions of microphyllous taxa.
Latitudinal Patterns and Preservational Biases. Floras and dinosaur-bearing formations were assigned
to 10⬚ paleolatitudinal bins and the taxa then
summed for each bin. An individual taxon was
counted only once per bin, regardless of its number
of occurrences. Results are shown in figure 2. In
the Northern Hemisphere, dinosaur diversity is
highest between 30⬚ and 40⬚N, but plant diversity
peaks slightly poleward, between 40⬚ and 50⬚N.
Plant and dinosaur diversity in the Southern Hemisphere peaks at 30⬚–40⬚S, although in both cases
diversity is lower than in the Northern Hemisphere. This is probably due to factors that include
less intensive sampling and smaller land surface
area in the Southern Hemisphere (fig. 1). Diversity
Journal of Geology
DINOSAUR DISTRIBUTIONS
is otherwise broadly symmetrical about the equator
(fig. 2), reaching a maximum in the midlatitudes
and a minimum in the low and high latitudes, although the highest levels of dinosaur diversity are
restricted latitudinally (30⬚–50⬚N), whereas highest
plant diversity spans a broader latitudinal range
(30⬚–70⬚N). The poleward limit of dinosaur fossils
is only 40⬚–50⬚N and S, whereas plant fossils extend
to 70⬚–80⬚N and 60⬚–70⬚S.
Although figure 2 shows latitudinal distribution
patterns, it reveals nothing about longitudinal variations. Dinosaur localities with the highest numbers of taxa are concentrated in Europe and North
America (fig. 1C). In contrast, genus-rich plant localities are concentrated in China, eastern Russia,
India, and Europe, with relatively few in North
America (fig. 1A). In the case of Europe, dinosaur
and plant localities appear to be concentrated together (fig. 1), at least in the southern part. However, compared with more northerly plant localities, the southern European ones have more
abundant microphyllous, dry-adapted forms of conifers and cycadophytes.
The absence or paucity of dinosaur fossils at
higher latitudes is in marked contrast to the abundance of fossil floras and coals. At lower latitudes,
the regional co-occurrence of dinosaurs with evaporites and floras containing abundant microphyllous taxa is also noteworthy. These may represent
real biological patterns, but first we consider the
effects of taphonomic bias. Several traits of the depositional environment will affect its ability to preserve vertebrate material including temperature,
precipitation, vegetation, and underlying geology,
which determine the chemical and physical properties of a soil.
Arid and semiarid climates result in longer residence times on the surface for vertebrate remains,
tending to increase time averaging and concentration of remains, whereas hard parts (e.g., bone) degrade more quickly in humid climates as a result
of biotic as well as abiotic processes (Behrensmeyer
et al. 2000). Increasing temperature can cause an
increase in biotic and chemical activity that may
be destructive to the remains (Hare 1980; Yavitt
and Fahey 1986). Both pH and Eh (oxidation-reduction) have been shown to be the most important
factors determining vertebrate preservation in a variety of depositional environments (Hare 1980; Gordon and Buikstra 1981; Retallack 1998). Precipitation affects bone preservation through its ability
to mobilize ions in the soil, leading to the leaching
of surface materials, especially organic acids (Yavitt
and Fahey 1986). This leaching is what leads to the
overall lower pH of forest soils and the eventual
647
dissolution and destruction of bone. Soils in more
humid climates also have a greater amount of organic material than in arid and semiarid areas.
Floodplain soils are drier, more mineral rich, and
have a lower organic content. Consequently, their
pH is higher than that of areas with more precipitation (Pritchett 1979). Vegetation plays an important role chemically, through the exchange of
materials (water, minerals, and nutrients) that affect soil chemistry, and physically, by the action of
roots (Henderson 1987; Berner et al. 2003). Plant
roots can adversely affect bone preservation
through soil leaching of mineral components as
well as physical degradation. Plants also play a role
in determining the nature of soils in which an animal is buried, and plants with more complex root
systems have been shown to dramatically increase
rock weathering rates (Cochran and Berner 1996;
Berner et al. 2003).
The interplay between these factors will ultimately be responsible for the destruction or preservation of an animal’s remains. This may explain
why Late Jurassic dinosaur bones are scarce to nonexistent in higher latitudes, where the prevailing
climates were generally warm and wet, albeit seasonal, with broad tracts of gymnosperm forests
dominating the landscape.
Case Studies of Late Jurassic DinosaurDominated Ecosystems
A detailed global analysis of the paleoecology of
Jurassic dinosaur-bearing deposits is beyond the
scope of this study. However, an examination of
floral and faunal patterns in two deposits yielding
abundant dinosaur remains, the Morrison Formation of North America and the Tendaguru beds of
Tanzania (fig. 1C), can offer some insights into the
kinds of conditions that favored diverse, sauropodrich faunas in the Late Jurassic. The dinosaurs,
plants, and paleoenvironments in both formations
have been studied intensively.
Similarities between the faunas of the Morrison
Formation and the dinosaur-bearing sediments of
the Tendaguru beds of Tanzania have long been
noted (Schuchert 1918; Russell et al. 1980; Maier
2003). The deposits both appear to span the
Kimmeridgian-Tithonian interval, and their fossil
records comprise plants, invertebrates, and a
dinosaur-dominated vertebrate assemblage. Some
of the greatest similarities are apparent in the dinosaur faunas, each of which is dominated, both in
diversity and abundance of individuals, by sauropods and also includes stegosaurs and small
ornithopods.
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P. M . R E E S E T A L .
The floras of the Morrison Formation have been
studied by numerous workers (see Ash and Tidwell
1998; Litwin et al. 1998; Parrish et al. 2004 for recent reviews). Those of the Tendaguru have been
studied by Kahlert et al. (1999), Schrank (1999), and
Grube et al. (2001).
Floras. Plant material is particularly sparse in
the Kimmeridgian part of the Morrison Formation
(Parrish et al. 2004). Microphyllous conifers (e.g.,
Brachyphyllum) are represented by five species, followed by ginkgophytes (e.g., Czekanowskia), cycadophytes, and sphenophytes, along with rare
ferns and bryophytes.
Parrish et al. (2004) considered the following as
possible explanations for the paucity of plant material in most environments: (1) complete mechanical degradation, (2) complete oxidation, and/or (3)
a depauperate source. Mechanical degradation by
physical and biological processes was clearly important in environments with strong currents or
abundant bioturbators, such as channels and lakes,
respectively. Oxidation was also likely an important mechanism, given the large volume of finegrained rocks that have no carbonaceous debris.
Most rocks from this interval are light greenish
gray, light to medium gray, red, tan, or purple; dark
gray rocks are rare. However, even those more reducing lithofacies contain very little plant material,
suggesting that, even when conditions were favorable for preservation, few plants were preserved.
This indicates that the vegetation was not particularly dense or large-statured in most places. That
the plants were mostly small in stature is supported
by the fact that evidence for deep rooting or large
roots is rare in the Morrison Formation (Hasiotis
and Demko 1998). The leaf mats of Czekanowskia
are the one exception to the general scarcity of
plant occurrences in the Kimmeridgian. They are
monospecific and limited to narrow intervals, suggesting that the ecology of this plant was different
from the others (Parrish et al. 2004).
In contrast to those of the Kimmeridgian, the
Tithonian-age rocks tend to be gray and brown, and
the red, green, and light brown colors that characterize the Morrison elsewhere are lacking. Plant
megafossils are more abundant than in the Kimmeridgian (Parrish et al. 2004) and comprise several
species of ferns and cycadophytes, along with macrophyllous conifers, ginkgophytes, sphenophytes,
pteridosperms, and bryophytes. This part of the formation also contains thin, high-ash, high-sulfur
coal beds (Calvert 1909; Fisher 1909) that were
mined mostly between 1885 and 1955 (Silverman
and Harris 1967). The sedimentologic evidence indicates that the Morrison in central Montana was
deposited in mires and associated rivers, floodplains, and lakes.
Unlike other thin coal-bearing units (e.g., the
Cretaceous Kogosukruk tongue of the Prince Creek
Formation, Alaska; Spicer and Parrish 1987; Parrish
and Spicer 1988), the Tithonian part of the Morrison Formation is depauperate of fossil wood. The
coal beds themselves contain little vitrinite, indicating that the mire community was also depauperate in woody plants (Parrish et al. 2004). However, the density of identifiable leaf remains on
bedding planes is higher in the Tithonian than in
the Kimmeridgian of the Morrison, although the
rootlets and rhizomes in these younger floras are
the same small size as in the rocks elsewhere in
the Morrison. This is consistent with the paucity
of wood and the low vitrinite content of the coals.
Whereas the Morrison Formation has been variously described as humid or arid, recent work suggests that the climate was predominantly semiarid,
with slight variations in moisture availability owing to climate and/or higher water tables (Demko
1998; Parrish et al. 2004). For example, Demko
(1998) described fluvial and floodplain deposits that
contain calcareous, vertic paleosols, as well as eolian sandstone, bedded gypsum, and lacustrine deposits that formed in a large saline-alkaline lake
(Turner and Fishman 1991).
The Tendaguru macroflora is best represented in
the Middle Saurian bed, which also includes the
most abundant dinosaur fossils. Cuticle analysis
indicates four families of gymnosperms: Taxaceae, Cupressaceae, Cycadaceae, and Ginkgoaceae
(Grube et al. 2001). The upper Saurian beds also
preserve a diverse cuticle flora, including Podocarpaceae and Cheirolepidiaceae. Palynological analysis of the entire section at Tendaguru indicates a
dominance of the Cheirolepidiaceae taxon Classopollis, with the Araucariaceae taxon Araucariacites also common. These are conifer pollen, and
it is noteworthy that the corresponding leaf fossils
(e.g., Frenelopsis, Brachyphyllum; Vakhrameev
1991) are typically microphyllous. The Middle Saurian beds also yield spores of ferns and bryophytes.
Sedimentology and Paleoenvironments. Parrish et
al. (2004) documented sedimentological characteristics of the Morrison Formation that are relevant
to a climate interpretation. These include: (1) eolian sandstone of the Bluff Sandstone and Recapture
Members in the southern part of the Morrison Formation and equivalent beds as far north as northcentral Wyoming and western South Dakota (Peterson 1988); (2) thick beds of gypsum of the
terrestrial and marginal-marine Tidwell Member;
(3) aridisols and gypsic entisols of the Tidwell and
Journal of Geology
DINOSAUR DISTRIBUTIONS
Salt Wash Members, calcisols and argillic calcisols
of the Salt Wash Member, vertic and argillic calcisols of the Brushy Basin Member, weakly developed entisols and calcisols in the upper Brushy Basin Member and equivalent rocks to the north, all
of which were in the Kimmeridgian part of the Morrison, and gleysols and histosols in the Tithonian
parts of the Morrison (Demko 1998); and (4) salinealkaline lake deposits, with characteristic zonation
of authigenic minerals, including zeolites of the
Brushy Basin Member (Lake Too’dich’i’; Turner and
Fishman 1991). As with the plants, the differences
between the Kimmeridgian and Tithonian characteristics of the Morrison are noticeable but not
large, and the differences are consistent with a
slightly greater supply of moisture, through higher
groundwater levels, lower evaporation rates, and/
or higher precipitation rates during deposition of
the upper part of the Brushy Basin Member and
correlative rocks elsewhere.
The succession of dinosaur-bearing beds in Tendaguru is similarly dominated by calcareous sandstones and siltstones, but their estuarine, coastal,
and shallow marine origin is demonstrated by the
abundance of marine invertebrate taxa including
dinoflagellates, corals, ammonites, and gastropods
(Aberhan et al. 2002). The vertical succession at the
dinosaur-bearing site captures a succession of transgressions dominated by marine taxa and regressions
that mix articulated and disarticulated dinosaurs
with the marine fauna. Calcrete horizons in the
dinosaur-bearing intervals indicate alternating wetdry conditions similar to those observed for the
Morrison.
Dinosaur Distributions and Feeding. A quantitative comparison of the abundance of various taxa
in the two deposits is instructive. In the Tendaguru,
the most abundant sauropod taxon is Brachiosaurus (Russell et al. 1980; minimum number of individuals [MNI] p 26), compared with an MNI of
11 observed for the second-most abundant taxon,
the smaller, low-browsing diplodocid Dicraeosaurus. In contrast, the Morrison fauna is dominated
by diplodocids (principally Diplodocus and Apatosaurus) and the sauropod Camarasaurus, with
Brachiosaurus a consistently rare element of the
fauna (Foster 2003).
The Morrison dinosaur fauna is dominated by
taxa that appear to be low to medium browsers,
based on computer modeling of feeding heights
(e.g., Stevens and Parrish 1999 and in press). The
presence of a marked ventral inclination of the
skull relative to the vertebral column in diplodocids is consistent with their occupation of a low
browsing guild (Parrish 2003). This interpretation
649
of sauropod feeding correlates with the abundance
of aquatic, herbaceous, and ground-covering plants
in the Morrison.
By contrast, the Tendaguru sequence, particularly the best-represented fauna of the Middle Saurian beds, is dominated quantitatively by the giant
sauropod Brachiosaurus (Russell et al. 1980). Although the morphology of the neck of Brachiosaurus is not completely known, and differences exist
in interpretation of the neck inclination of this animal (e.g., Paul 1988; Christian and Heinrich 1999;
Stevens and Parrish, in press), different researchers
agree that this animal was probably the highestbrowsing member of the Late Jurassic sauropod faunas. This is consistent with the abundance in the
Tendaguru of Cheirolepidiaceae and Araucariaceae
as both pollen and cuticle. The presence of lowbrowsing taxa such as the diplodocid sauropod Dicraeosaurus and the stegosaur Kentrosaurus in the
Tendaguru beds suggests that, as in the Morrison,
these animals may have been feeding on low- to
medium-height plants such as bryophytes, ferns,
and sphenophytes (e.g., Equisetum).
In summary, the Morrison and the Tendaguru
dinosaur-rich deposits occur at similar paleolatitudes but in different hemispheres. They appear to
represent semiarid paleoenvironments with the
greatest abundance and diversity of herbaceous and
arborescent plants concentrated near bodies of water. Significant family and genus-level differences
in the dinosaurs present in the two deposits may
relate to differences in the structure of their plant
communities.
Conclusions
The exact nature and spatial distribution of vegetation and environments present in the Morrison
and Tendaguru ecosystems is difficult to interpret
conclusively; however, it can be seen that an immense biomass and diversity of dinosaurs was apparently supported by sparse vegetation. Modern
tropical savannas, which are composed of grasses
and scattered shrubs and trees, with a higher density of shrubs and trees along the water courses,
are the obvious choice for comparison with these
extinct ecosystems. Although the plant groups in
these ecosystems are different (e.g., Jurassic sphenophytes and ferns vs. modern grasses), they share
many characteristics, such as broad, open areas of
low-standing vegetation surrounding concentrations of arborescent plants near bodies of water.
This spatial arrangement may determine the diversity and biomass of herbivores in an ecosystem
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P. M . R E E S E T A L .
more than the potential productivity of the vegetation (Taggart and Cross 1997).
Modern tropical savannas are broad summer wet
zones transitional between deserts and humid forests, where the growing season is controlled by alternating wet and dry periods (Bourlière and Hadley
1983). The Late Jurassic Morrison and Tendaguru
formations occupied a winter wet biome, between
desert and warm temperate biomes (see Rees et al.
2000 for maps and biome descriptions). We emphasize that these Late Jurassic “savannas” occurred in midlatitudes and were bounded equatorward by desert belts; they do not, therefore, indicate
poleward expansion of the tropical savanna biome.
However, conditions were seasonally wet and temperatures relatively high, so a comparison with
modern summer wet savannas is valid. Compared
with forested ecosystems, tropical savannas can
have a higher net productivity during the wet season if soil fertility is not limiting. Much of this
biomass is readily available because it is more palatable and easily acquired as leaves or seasonally
regenerating shoots, in contrast to the large
amounts of relatively unattainable wood that
makes up the majority of biomass in forests (Bourlière and Hadley 1983). Seasonal productivity leads
to herbivores (e.g., African elephants) that are nomadic or long-range migrants.
The tropical savanna model fits well with reconstructions of the Morrison environments (Dodson
et al. 1980; Engelmann and Fiorillo 2000). Some of
the confusion surrounding descriptions of the Morrison climate may be due to the time-averaging
and/or differential preservation of sediments from
wet and dry seasons. Recent work by Dunagan
(2000) shows that Morrison paleoclimate was at
least semiarid and perhaps at times even intermediate between semiarid and subhumid. Late Jurassic savannas may have covered a considerable
area, allowing herds of sauropods and other large
dinosaur herbivores to roam vast distances in
search of food, through drier areas to more riparian
settings. Aggregation of various species around water sources is not uncommon today and may partially explain the abundance of fossils in the Morrison that have been recovered from lacustrine,
fluvial, and adjacent floodplain deposits (Engelmann and Fiorillo 2000).
Modern tropical savannas are important centers
of ecological diversity. A relatively simple trophic
structure allows small perturbations to lead to
changes in the dominant species. Species turnover
due to difficult conditions favors rapid evolution,
possibly acting as a “species pump,” distributing
new species to surrounding habitats. Over geologic
time, xeric habitats such as savannas may have supported more species than mesic ones (Bourlière and
Hadley 1983). In this regard, pregrassland savannas
may have been a significant vegetation type during
the Mesozoic.
Vegetation and climate reconstructions indicate
that the Late Jurassic lacked a tropical everwet biome, which is associated in today’s world with high
biodiversity. Instead, Late Jurassic midlatitude and
high-latitude regions supported high-diversity forest communities growing in temperate climates.
However, the dinosaur remains are known mostly
from areas that occupied only seasonally wet biomes at lower latitudes, where vegetation was relatively sparse and of lower diversity. Dinosaurs did
exist at higher latitudes at other times during the
Mesozoic. They were most likely smaller than
those of the “savannas” because of size restrictions
placed on forest-dwelling animals and may have
had lower taxonomic diversity due to the relative
inaccessibility of a large portion of the forest biomass. In contrast, relatively dry environments as
in savannas tend to favor large herbivores (Engelmann et al. 2004). Coupled with taphonomic conditions that would have inhibited vertebrate preservation, the high-latitude record of Late Jurassic
dinosaurs remains sparse.
ACKNOWLEDGMENTS
We are indebted to F. Ziegler for his advice, encouragement, and friendship over the years and for
his ideas during the early stages of this work. We
thank C. Scotese for the use of his plate reconstruction software and D. Rowley for discussions and
help with other global-scale aspects of this study.
We also thank two anonymous reviewers for their
insightful comments.
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